In recent years, it has been shown that cancer is a genetic and epigenetic disease, where epigenetic and genetic alterations interact reciprocally to drive cancer development. However, unlike genetic mutations, epigenetic changes are reversible, and as such, drugs that restore the epigenetic balance represent exciting potential therapeutic targets for cancer. Epigenetics refers to the heritable changes in gene expression patterns that occur independently of alterations in primary DNA sequence. The main epigenetic mechanisms are DNA methylation and covalent histone modifications, which play important roles in the regulation of transcription.
DNA methylation is an epigenetic modification that modulates gene expression without altering the DNA base sequence and plays a crucial role in cancer by silencing tumor suppressor genes. DNA methyltransferases (DNMTs) are the enzymes that catalyze DNA methylation. DNMT1 encodes the maintenance methyltransferase and DNMT3A and DNMT3B encode de novo methyltransferases.
DNMT1 and DNMT3A/3B are overexpressed in several types of cancer such as breast, gastric, pancreas, prostate, hepatocellular, ovarian, renal, retinoblastoma, glioma or diffuse large B-cell lymphoma. The DNA hypomethylating agents like Zebularine, decitabine and azacytidine inhibits cell proliferation and induce apoptosis in acute lymphoblastic leukemia, acute myeloid leukemia, hepatic carcinoma, lung, breast, gastric or cervical cancer among others (Vilas-Zornoza A. et al., PLoS ONE 2011, 6(2): p. e17012). Decitabine has been currently approved for myelodysplastic syndrome by the US Food and Drug Administration. On the other hand, DNA methylation plays a key role in the pathogenesis of fibrosis (Neary, R. et al, Fibrogenesis & Tissue Repair 2015, 8:18). Further, DNA methyltransferase inhibition also accelerates the immunomodulation and migration of human mesenchymal stem cells (Lee S. et al., Scientific Reports 2015, 5:8020).
However, many efforts are made to develop new non-nucleoside inhibitors to overcome the limits of these azanucleosides, such as chemical instability and incorporation into DNA for activity.
G9a, also known as EHMT2, is a histone methyltransferase that mono- and dimethylates Lysine 9 of histone H3 (H3K9me1 and H3K9me2, respectively). G9a expression is high in many cancers compared with normal tissue. Cancer transcriptome analysis has revealed high expression in many tumors including hepatocellular, colon, prostate, lung bladder and invasive transitional cell carcinomas and in B cell chronic lymphocytic leukemia (Shankar S R. et al., Epigenetics 2013, 8(1): p. 16-22). Knockdown of G9a in both bladder and lung cancer cell lines caused growth suppression and apoptosis. Studies on prostate cancer further corroborate its role in carcinogenesis, where downregulation of G9a causes centrosome disruption, chromosomal instability, inhibition of cell growth and increased cellular senescence in cancer cells. In aggressive lung cancer, high levels of G9a correlate with poor prognosis with increased cell migration and invasion in vitro and metastasis in vivo. G9a is also overexpressed in pancreatic adenocarcinoma and inhibition of G9a induces cellular senescence in this type of cancer. In Acute Myeloid Leukemia mouse models, loss of G9a significantly delays disease progression and reduces leukemia stem cells frequency.
Interestingly, DNA methyltransferase-1 (DNMT1) physically interacts with G9a to coordinate DNA and histone methylation during cell division (Esteve P O. et al., Genes Dev 2006, 20:3089-3103) promoting transcriptional silencing of target genes (Tachibana M. et al., EMBO J 2008, 27:2681-2690). In this sense, reduction of both DNA and H3K9 methylation levels leads to reactivation of tumor suppressor genes and inhibits cancer cell proliferation (Wozniak R J. et al., Oncogene 2007, 26, 77-90; Sharma S. et al., Epigenetics Chromatin 2012. 5, 3 (2012).
There is still a need of developing compounds which show improved activity in the treatment and/or prevention of cancer, fibrosis and immunomodulation.